Thermoplastic resins are used in a multitude of commercial applications. Polyesters such as polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and similar polymers and copolymers, in particular, have become staple commodities whose manufacture is well known and mature. Applications of polyesters include food, beverage, and other liquid containers as well as synthetic fibers. Several polyesters such as PET may exist both in amorphous and semi-crystalline forms. Amorphous PET is transparent while crystalline PET is opaque.
In the conventional PET process, PET is formed by esterification of terephthalic acid and ethylene glycol in a reaction vessel to form a pre-polymeric mixture. The esterification need not be catalyzed. The pre-polymeric paste is subsequently heated to promote polymerization. The resulting mixture is then subjected to polycondensation in a melt at elevated temperatures, for example, 285° C., in the presence of a suitable catalyst. Compounds of Sn, Sb, Ge, Ti, or others have been used as polycondensation catalysts. The polymer is extruded directly from the polycondensation reactor into strands. The hot, extruded strands are contacted with cool water prior to chopping into pellets, dried, and stored into silos prior to crystallizing.
Pelletizing processes wherein strands are stretched prior to pelletizing are disclosed in U.S. Pat. No. 5,310,515. Conventional wisdom dictates that at least the surface of the pellets must be cooled to 20° C. to 30° C. to avoid sintering during storage. During storage, heat from the hotter interior of the pellets is distributed throughout the pellets. Thus, warm pellets, i.e., pellets whose exterior is significantly higher than 20° C. to 30° C. might agglomerate during storage following temperature equilibration. In addition to the decrease in temperature brought about by contact with water, the pellets can be further cooled to the desired temperature with cool air, nitrogen, or inert gas. The pellets are stored, and then subsequently reheated to the desired crystallization temperature. These steps of heating, cooling, and reheating result in a significant energy penalty in an already energy intensive process. The crystallization of the hot pellets can be accomplished in a crystallizing shaker or fluid bed. Solid stating is used to both raise inherent viscosity and remove acetaldehyde.
With reference to FIGS. 1A, 1B, and 1C, diagrams of PET manufacturing facilities are provided. PET processing facility 10 includes mixing tank 12 in which terephthalic acid (“TPA”) and ethylene glycol (“EG”) are mixed to form a pre-polymeric paste. This pre-polymeric paste is transferred and heated in esterification reactor 14 to form an esterified monomer. The pressure within esterification reactor 14 is adjusted to control the boiling point of the ethylene glycol and help move the products to esterification reactor 16. The monomer from esterification reactor 14 is subjected to additional heating in esterification reactor 16 but this time under less pressure than in esterification reactor 14. Next, the monomers from esterification reactor 16 are introduced into pre-polymer reactor 18. The monomers are heated while within pre-polymer reactor 18 under a vacuum to form a pre-polymer. The inherent viscosity of the pre-polymer begins to increase within pre-polymer reactor 18. The pre-polymer formed in pre-polymer reactor 18 is sequentially introduced into polycondensation reactor 20 and then polycondensation reactor 22. The pre-polymer is heated in each of polycondensation reactors 20, 22 under a larger vacuum than in pre-polymer reactor 18 so that the polymer chain length and the inherent viscosity are increased. After the final polycondensation reactor, the PET polymer is moved under pressure by pump 24 through filters 26, 28 and through dies 30, 32, 34, forming PET strands 36, 38, 40 (see FIG. 1B).
With reference to FIG. 1B, a method for forming polyester pellets is illustrated. Extruded polymer strands 36, 38, 40 are cooled by water spray streams 42, 44, 46 onto the strands as the strands emerge from dies 30, 32, 34. After emerging from dies 30, 32, 34, strands 36, 38, 40 are cut by cutters 54, 56, 58 into pellets 48, 50, 52 while the strands are still hot. Polyester pellets formed in this manner tend to have a cylindrical shape, but can be modified to cubic, dog bone, or other shapes. At this point in the process, polyester pellets are usually amorphous. The polyester pellets are typically crystallized before being shipped to a customer. Such crystallization allows subsequent drying at higher temperatures so that the polyester may be extruded as desired. Crystallization of the polyester pellets is typically achieved by reheating the pellets to a temperature above the crystallization temperature. As the pellets crystallize, additional heat is derived due to the generated heat of crystallization. This additional heat tends to make the pellets soft and adherent to each other. Therefore, the pellets are agitated to avoid them sticking together due to softening. After crystallization, the pellets are generally solid stated to raise inherent viscosity with inert gas passing around the hot pellets.
With reference to FIG. 1C, a schematic of an alternative pellet forming process is provided. In this variation, strands 60, 62, 64 emerging from die dies 66, 68, 70 are cut into pellets 72, 74, 76 under water by die face cutters 80, 82, 84. In this variation, the extruded polyester strands are completely immersed and cut underwater upon exiting dies 66, 68, 70. Pellets 72, 74, 76 formed in this manner tend to have a spherical shape because of the surface tension of the molten polyester when emerged in water. Initially, after cutting, pellets 72, 74, 76 still retain a substantial amount of heat in the interior. Subsequently, the pellet/water combination is sent to dryer 90 via conveying system 92. Examples of useful dryers include centripetal dryers that remove pellets 72, 74, 76 from the water. Upon exiting dryer 90, additional water is boiled off due to the heat content of pellets 72, 74, 76, which is still high upon emerging from dryer 90. If the pellet/water combination is transported to the dryer sufficiently fast the polyester pellets may retain sufficient heat for crystallization to occur. Pellets 72, 74, 76 are then transferred to crystallizer 94 where they reside for a residence time (about 2 to 20 minutes) for crystallization to occur. Crystallizer 94 also provides sufficient agitation to inhibit the polyester pellets from sticking together.
International Patent Appl. No. WO2004/033174 and U.S. Pat. Appl. Nos. 20050110182 and 20050110184 disclose methods for crystallizing polymeric pellets. International Patent Appl. Nos. WO2004/033174 discloses a method in which polymeric pellets are treated in a liquid bath (e.g., water bath) at an elevated temperature to induce crystallization. U.S. Pat. Appl. Nos. 20050110182 and 20050110184 disclose method in which air is injected into the pellet/water slurry of FIG. 1C in order to transport the pellets quickly to and through dryer 90.
After crystallization, pellets 72, 74, 76 are transported by dense phase convey system 96 to one or more pellet processing stations. Such dense phase convey systems utilize air to move the pellets from one location to another. For example, the pellets are transported to a blending silo in which the average properties of the pellets might be adjusted. In such blending silos, polyester pellets are mixed together to achieve a target specification. Such specification may be with respect to color, molecular weight, catalyst concentration, additive concentration, density, and the like. In still another example, the pellets are conveyed to a solid stating process reactor. It should be noted, that dense phase convey systems tend to be more useful than dilute phase convey systems in this application since dilute phase convey systems can result in the surface of the pellets being melted or have high impact velocities thereby forming undesirable streamers and fines.
Although these methods and systems for making polymeric pellets and, in particular, polyester pellets work well, the equipment tends to be expensive to fabricate and to maintain. A typical PET manufacturing line may include several crystallizers each of which utilizes a rather large motor and occupies a large footprint in the manufacturing plant. The initial capital investment of such crystallizers may easily exceed a million dollars.
Accordingly, there exists a need for polymer processing equipment and methodology that is less expensive to install, operate, and maintain.